Electrochemical cells may be classified as primary or secondary. Primary cells are those that derive electrical energy from a chemical state, and are those whose electrodes are generally not rechargeable. Examples of primary battery systems are those having as electrodes mercury-zinc; silver-zinc; lead-zinc; copper-zinc; copper-magnesium; and silver-magnesium. Secondary cells are basically electrical energy storage cells, and are rechargeable electrically by passing a current through the cell in a direction reversed from that of discharge. Illustrative of secondary battery systems are those having as electrodes, nickel-cadmium; silver-zinc and silver-cadmium.
In either case, the cell is made up of two half-cells, each comprising an electronic conducting phase or electrode in contact with a second phase called an electrolyte in which ionic conduction takes place. A common electrolyte used in both primary and secondary cells is a 30 to 40% solution of KOH. The electrolyte associated with the cathode is referred to as the catholyte and that associated with anode is the anolyte. In some cells, the catholyte and anolyte are different solutions and therefore require a separator membrane to prevent the two solutions from physically mixing. In other cells, the catholyte and anolyte are the same, in which case the separator functions to physically separate the cathode and anode.
Alkaline secondary electrochemical cells are extremely valuable for various commercial, military and aerospace applications. However, this apparent advantage is offset by the disadvantage of limited cycle life. For example, in common secondary alkaline electrochemical cells in which silver is the positive electrode, the transmigration of silver oxides dissolved or suspended in the alkaline electrolyte to the negative electrode results in local couples and self-discharge of the negative plate. Also, in secondary alkaline electrochemical cells in which zinc is an electrode, zinc dendrites deposited on the negative plate during charge, as a result of the reduction of potassium zincate in the alkaline electrolyte, rapidly bridge the narrow gap between the cathode and the anode, thereby short-circuiting the cell.
In the past, battery engineers have sought to obviate these disadvantages through the use of various types of separator membranes. In order to be effective, the separator membranes must possess certain physical, as well as chemical properties, such as low electrolytic resistance; low resistance to hydroxyl migration and high resistance to silver oxide migration; and high resistance to oxidation, particularly in alkaline solutions at high temperatures. Furthermore, the membrane must possess sufficient mechanical strength to withstand the rigors of battery assembly and to prevent zinc dendrite growth or treeing between the cathode and anode.
Then known separator membranes, such as microporous and cellulosic materials, did not possess these physical and chemical properties and proved to be unsatisfactory for use in secondary alkaline electrochemical cells, especially for those having silver electrodes.
To obviate these disadvantages associated with the use of known separator membranes battery engineers developed membranes with improved characteristics by permanently bonding ionizable groups, such as a carboxyl and sulfonic acid groups, onto an inert polymer film using irradiation grafting techniques. Membranes of this type and procedures for their manufacture are disclosed in U.S. Pat. Nos. 4,201,641, 3,427,206 and 4,012,303. While such membranes are relatively effective, they too have several disadvantages, the most significant of which result from the methods employed to prepare them.
For example, in the known irradiation grafting procedures, the solvents used are aromatic or halogenated hydrocarbon solvents, such as benzene, carbon tetrachloride and methylene chloride. The use of such solvents is disadvantageous in that certain of them are health hazards because of high flammability and because such solvents can be toxic to the user. In addition, these solvents are difficult to dispose of after use because they are harmful to the environment. Further, in recent years, the cost of such solvents has increased significantly, resulting in a concomitant increase in the cost of the separator membranes prepared by processes which employ such solvents. In other known procedures, as for example that described in U.S. Pat. No. 4,201,641, mixtures of one of the aforementioned hydrocarbon solvents and water are used. In addition to the above-mentioned disadvantages, this process requires large amounts of the grafting monomer, i.e., greater than 30% by volume, or otherwise aqueous and organic solvent components tend to separate into layers. Also in this process, the organic solvent is a critical component or otherwise the the resistivity of the separator membrane will be unacceptably high. Thus, the net result is that larger amounts of the grafting monomer must be used, which causes an increased likelihood of waste of the monomer reactant.
Certain of these known irradiation procedures also suffer from the defect that a homopolymer of the grafting monomer is formed during the conduct of the irradiation procedures. The homopolymerization side reaction depletes the amount of monomer available for grafting, and results in a non-homogenous graft separator membrane. Furthermore, the homopolymer adheres to the separator membrane relatively strong, and is difficult to remove. The adhering homopolymer reduces the usefulness of the separator membrane by increasing its resistance as much as 1000 percent.
It is therefore an object of this invention to provide an irradiation grafting process for preparing an improved separator membrane for use in primary and secondary electrochemical cells which has low electrolytic resistance, high ohmic resistance and low resistance to hydroxyl migration, but high resistance to silver oxide migration.
It is a further object of this invention to provide an irradiation grafting process for preparing a separator membrane which is resistant to oxidative degradation and hydrolytic attack in electrolyte solutions, particularly at high temperatures.
It is also an object of this invention to provide an irradiation grafting process for preparing separator membranes, in which homopolymerization of grafting monomer is either eliminated or greatly retarded.
It is still another object of this invention to provide an irradiation grafting process for preparing separator membranes which employ water, rather than aromatic and halogenated hydrocarbon solvents and aqeous mixtures thereof as the process solvent.
It is yet another object of this invention to provide an irradiation grafting process for preparing separator membranes in which the percent graft is not sensitive to the radiation dose rate, and high percent grafts can be obtained with short term, high dose irradiation.
Other objects and advantages will be apparent from the following disclosure and appended claims.